RFID uses electromagnetic fields to automatically identify and track tags attached to objects. The tags contain electronically stored information. RFID tags are used in many applications such as security cards, inventory control and storing small amounts of information. In their simplest form, passive RFID tags consist of an antenna connected to an integrated circuit (IC). The reader is an external device that transmits radio waves towards the RFID tag and measures the backscattered wave amplitude versus time. The antenna transfers the incident radio waves to the IC which modulates the impedance seen by the antenna versus time according to the stored data. When the impedance seen by the antenna is close to its input impedance (e.g., 50 Ohms), the backscattered wave amplitude is very low. On the other hand, when the impedance seen by the antenna is close to zero (short circuit) or very high (open circuit) the backscattered wave amplitude is high. The reader detects this amplitude and uses it to determine the stored information or the tag number of the IC.
RFID ICs are fabricated using silicon technology and require additional packaging and assembly to attach to the antenna. This results in higher cost and limits the use of RFIDs to relatively higher cost systems. An alternative is connecting the antenna to multiple length transmission lines which provide multiple delays corresponding to the stored data inside the RFID. Unfortunately, due to the very fast propagation speeds of radio waves, very long transmission lines such as hundreds of meters are needed to achieve significant delays that are easy to detect which results in a very large or expensive system.
Typical delay lines are implemented using discrete components or electrical transmission lines, which results in a relatively large circuit if the delays required are long.